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A discussion of Radiative Transfer Models

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**1. **A discussion of Radiative Transfer Models Thomas J. Kleespies
NOAA/NESDIS

**2. **Disclaimer There will be some overlap between this presentation and this afternoon’s presentation
There will be some overlap between these two presentations and other speakers
This is good!
This presentation will be a broad brush overview of radiative transfer models relevant to data assimilation (i.e. fast and with adjoint)

**3. **Outline Scattering vs absorption models
Line vs fast models
Generally available line models
Generally available fast models
Generally available scattering models
Generally available general purpose models

**4. **Scattering vs Absorption models Full radiative transfer equation
Surface emission
Atmospheric emission
Surface reflection
Downward atmospheric
Downward solar
Downward CBR
Single scattering
Multiple scattering
Solar and Thermal

**5. **Single vs Multiple Scattering

**7. **General Types of Absorption Models Line – by – Line models
Numerically integrate individual absorption lines to produce optical depth/transmittance profile as a function of wavelength
Band Models
fit to LBL at narrow spectral interval
Fast Models
Statistical fit to line-by-line model transmittance for diverse atmospheres for specific instrument bandpass

**8. **General Types of Scattering Models Discrete Ordinate Models
Radiative intensity not a function of azimuth
N streams … bigger N, better result, more expensive
Adding/Doubling Models
Form of ray tracing with known transmission/ reflection of each layer.
Reflection and transmission of combined layers obtained by computing successive reflections between two layers.
If layers have same optical depth, referred to as doubling
Monte Carlo

**9. **Additional Generally need a Mie code (spherical scatterers) or something like it to specify the scattering parameters
Need to pick a polydispersion of scatterers

**10. **Multiple scattering Models generally too expensive for data assimilation
When scattering is included in data assimilation RTM, it is generally single scattering, or a few streams approximation.
This is the topic of very active research

**11. **Generally Available Scattering Models DISORT
Discrete ordinate, N-stream plane parallel code
Fu-Liou
2 and 4 stream radiative transfer solver
SHDOM
1,2,3D RTM , including adjoint

**12. **Steps in absorption/emission modeling Spectroscopic Data Base
Absorber, line position, line strength, line half-width, etc.
Line model
Integrates individual lines over instrument spectral response.
Very expensive in infrared, almost trivial in microwave
Fast model
polynomial fit to LBL transmittances

**13. **Public molecular absorption databases HITRAN – Rothman et al.
HITRAN is a compilation of spectroscopic parameters that a variety of computer codes use to predict and simulate the transmission and emission of light in the atmosphere. The original version was created at the Air Force Cambridge Research Laboratories (1960's). The database is maintained and developed at the Harvard-Smithsonian Center for Astrophysics in Cambridge MA, USA.
GEISA - Jacquinet-Husson et al.
GEISA is a computer-accessible spectroscopic database, designed to facilitate accurate forward radiative transfer calculations using a line-by-line and layer-by-layer approach. Maintained and developed at Laboratoire de Météorologie Dynamique (LMD) in France.

**14. **Absorption Lines near 15 mm

**15. **Absorption Lines Near 4.3 mm

**17. **Microwave line absorption

**18. **Generally Available Line Models LBLRTM – AER (Clough et al.)
Based on FASCODE from AFGL
Supported by ARM
Widely used in US
Uses HITRAN
GENLEN2 – Edwards – NCAR
Used in Europe
Europeans use GEISA or HITRAN

**19. **Generally available band models MODTRAN
developed by AFGL
enhanced by private industry
based upon FASCODE/LBLRTM/HITRAN
extremely flexible in atmosphere, absorbers, geometry
extremely cumbersome data input
no adjoint available, too slow for DA
DoD support waning

**20. **Generally Available Fast Models Community Radiative Transfer Model-CRTM (OPTRAN)
used at NCEP/EMC, NASA,…
Radiative Transfer TOVS (RTTOV)
used at EUMETSAT, METOFFICE, others
AESFAST
used at Environment Canada
Optimal Spectral Sampling (OSS)
developed at AER, receiving some attention here

**21. **Why fast models? Line-by-line models can take up to an hour to compute a single channel/atmosphere
Fast models take maybe a millisecond
Line-by-line models don’t have adjoints, have to do finite differencing
Fast models have analytic adjoints. We will see how to write them this afternoon.

**22. **CRTM Based upon Optical Path TRANsmittance (McMillin et al)
Includes thermal emittance, clouds, simplified scattering, CBR, solar influence, surface emissivity models, support for many instruments
Yong Han will discuss in detail on 8 Aug

**23. **OPTRAN Predicts absorption coefficient on the absorber level (product of which is optical depth, negative exponential of that is transmittance) – arbitrary input profile
Zenith angle implicitly included in optical path
Fitted to LBLRTM/HITRAN and Leibe(89,93)
Adjoint well developed

**24. **RTTOV Predicts optical depth on fixed pressure levels – must interpolate input profile to the fixed levels
Must explicitly treat zenith angle
Includes thermal emittance, clouds, simplified scattering, CBR, solar influence, surface emissivity models, support for many instruments
Fitted to GENLN2/MPM89-92/HITRAN (Kcarta for IASI)
Adjoint well developed

**25. **Which is better? Depends on the situation/instrument
For a bit dated but still useful intercomparison of RTMs, see
http://collaboration.cmc.ec.gc.ca/science/arma/intercomparison/index.html

**26. **CRTM vs RTTOV A friendly competition is very healthy, and results in improvements in both codes
Institutions generally like to use locally developed codes because of control issues.

**27. **Innovation This is the de-biased difference between the radiance observations and the radiances computed from the NWP model background
Following statistics are from global assimilation systems, clear only.

**28. **NCEP Temperature

**29. **Metoffice Temperature

**30. **NCEP Water Vapor

**31. **Metoffice Water Vapour

**32. **ITWG Intercomparison

**33. **ITWG Intercomparison

**34. **Summary Overview of radiative transfer modeling
Emphasis on fast forward models
This afternoon I will discuss how the Jacobians are computed
You will have the opportunity to perform simple coding exercises to generate tangent linear/adjoint-jacobian code